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Comment by jacquesm | Hacker News Reader

NARS but probably comparable to shuttle engines and body.

The shuttle's main engines were extremely high-performance. They were marvels of engineering but required a lot of work between flights. Just like cars can be optimized for performance or reliability (think Honda Civic versus Ferrari) rocket engines can be made more maintainable and tolerant to long use periods. Not saying Spacex engines are Hondas, but they are probably way less trouble than SSMEs.

Soyez has been flying nearly 50 years. It's the dump truck of space, with 1700 launches to date. Compare that to the much more elaborate Shuttle's 135. Elon would rather have the dump truck than the Ferrari.

The Shuttle engines operate on hydrogen which burns very cleanly. RP-1 will leave the engine dirty with coking, so they definitely need to clean re-assemble each engine.

On the other hand, the solid boosters of the Space Shuttle were severely damaged by the impact and corrosion of the sea water, after "recovery" they were essentially a source of parts for new boosters.

Overall, I really hope they can improve re-usability above what the Space Shuttle achieved.

FWIW, SpaceX has launched stages after full-length (3 minute plus) burns on the test stand, on a schedule that wouldn't allow time for that kind of teardown. They'll need to do something to clean the engines out eventually, but if several minutes of operation were enough to cause serious gunking, then between those test runs and the multiple runs of the grasshopper test vehicle for landing sequences, they've seen it and know how to deal with it.

(Incidentally, SpaceX is known for keeping some aspect of their operations trade secret, rather than patenting. So, if they had some solvent that dissolved the gunk and simplified the cleanup process, or some such similar trick, it's likely that no one outside the company would know about it.)

The first stage goes through at least one full mission duration burn and another ignition before finally launching. So they don't need to be reassembled completely between each firing. I assume they have been repeatedly firing engines for a while now to see how many relights they can get.

The hydrogen in the Space Shuttle Main Engine creates embrittlement in the alloys of the engine. [1][2] The launch sequence also involved running the engines at 104.5% of rated capacity. [3] Before Block II of the engines, the turbopumps needed to be disassembled and rebuilt between flights. [4]

I have no real idea how significant coking of the Merlin engine is, but the SSMEs were hardly "clean".

[1] https://en.wikipedia.org/wiki/Hydrogen_embrittlement [2] http://www.tms.org/Superalloys/10.7449/1991/Superalloys_1991... [3] http://www.interspacenews.com/FeatureArticle/tabid/130/Defau... [4] http://www.nasa.gov/sites/default/files/174534main_ssme.pdf

IIRC the 104% was additional performance they enabled through upgrades and design improvements beyond the original design not over-driving them.

That right. The 100% level was set at some point during the design process. Later they found ways to slightly increase the thrust. Rather than recalibrate all the numbers, they just kept using the original "100%" and called the new level "104%."

Basically, it's "104% of what the engineers circa 1975 thought the level would be," not 104% of what the hardware was rated for.

They did re-light the engines, curious how many times in a row they can pull that trick before contamination becomes a problem.

The first stage rocket doesn't go nearly as high or as fast as the shuttle though, no? AFAIK the shuttle had to have its heat shield replaced after each return trip, and it landed aerodynamically rather than via rocket power. I'd presume that the powered descent of the first stage means that it doesn't need any sort of heat shielding, at the expense of more stress on the engines, thus requiring a bespoke inspection process.

It doesn't have a heat shield because it simply doesn't go fast enough relative to the atmosphere to need one, it does not 're-enter' the atmosphere at nearly the same horizontal speeds the shuttle did (from full orbital velocity down to landing speed). Even so, the stresses from heating up, cooling down, launch and landing are such that they will likely magnaflux that whole thing from one end to the other just to be sure there aren't any defects, especially until they have more experience, they'll pick it apart and inspect each and every bit to see how well it performed. Later on when they have much more data they might decide to swap out parts only every so many launches if possible.

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